Revert "Removing .html page extensions to simulate .htaccess"

This didn't actually work when pushed to GitHub Pages. When
clicking on links to the relevant pages they were downloaded,
instead of displayed.

This reverts commit 7bea0040b1e86bed7f9e54f6cd764d04a5890bc1.
Fixing links to "Getting Started" page

.html extension apparently is needed in GitHub Pages. It appears
that pages without that extension will be downloaded instead of
displayed in the browser.